1. Field of the Invention
The present invention pertains to liquid distributors for vapor-liquid contact towers and, more particularly, to an improved liquid distributor assembly for columns incorporating counter-current, vapor-liquid flow therethrough.
2. History of the Prior Art
It is well known in the prior art to utilize various types of exchange columns in which a gas and a liquid come into contact with one another, preferably in a counter-current flow for purposes of mass or heat transfer, close fractionation and/or separation of feed stock constituents, and other unit operations. Efficient operation requires mass transfer, heat transfer, fluid vaporization and/or condensation, whereby one of the fluids can be cooled with a minimum pressure drop through a particular zone or zones of minimum dimensions defining the area and volume thereof. These are pre-requisites for efficient operation and are necessary for close fractionation. For this reason, counter-current flow of vapor and liquid within such exchange columns have become established methods of such vapor-liquid contact in the prior art. The actual vapor-liquid interface requires the utilization of a packing bed positioned within the column. Liquid is then distributed atop the packing bed in the most feasible manner while vapor is distributed beneath the packing bed in the lower region of the tower. In this manner, liquid trickling downwardly through the packing bed is exposed to, and in contact with, the vapor ascending therethrough for vapor-liquid contact and interaction.
It is well established that the configuration of the packing bed determines the pressure drop, the efficiency of the vapor-liquid interface and the concomitant mass and energy transfer occurring in a process tower. The means for effective and even distribution of the vapor and the liquid on opposite ends of the packing bed as well as the means for maintenance of that distribution therethrough are critical to an efficient operation. Only with efficient initial vapor and liquid distribution and with the maintenance of such distribution throughout the packing bed will homogenous mixing zones be created therethrough for maximizing the efficiency therein. Efficiency in a column is directly related to the cost of operation and the production quality. For this reason, a myriad of prior art packing designs have been prevalent in conventional exchange columns. The efficiency of the packing is, however, limited to a large extent by the efficiency of the vapor and liquid distribution thereacross. For example, failure of either vapor or liquid to evenly distribute over cross sections of the packing effectively eliminates the utility or usefulness of that part of the packing where there is poor or no distribution. This phenomena, in turn, directly reduces the efficiency of the tower and decreases the cost effectiveness of the operation. The depths of the packing bed are critical in establishing production criteria and can affect the operational costs. Failure to evenly distribute vapor-liquid and/or maintain homogeneity within the packing bed can lead to serious consequences, particularly in the petroleum refining industry.
Aside from the packing beds themselves, the liquid distributor is the most important unit of a tower internal. Failure in performance of a packed tower sometimes stems from liquid distribution problems such as clogging or uneven distribution. Thus, the selection of a correct liquid distributor is critical for uninterrupted plant operation. Operational considerations thus include the functional aspects of the distributor, such as how level the distributor troughs are maintained, how well the floor is equalized therethrough, and how the liquid is distributed to the troughs so that a substantially uniform liquid level is maintained. The latter point is particularly true in process towers of large diameter where the parting box and distributor troughs are themselves very long and liquid gradients have been established between opposite ends of the troughs.
Conventional liquid distributors generally include the multi-orifice spray head variety adapted for dispersing liquid in the form of a spray atop a packing bed. In the utilization of dump packing wherein a plurality of random oriented packing elements are disposed within the exchange column, such a liquid distribution technique, is sometimes effective. This is true particularly when high efficiency parameters are not of critical significance. However, in the event of high efficiency packing such as that set forth in U.S. Pat. No. 4,604,247 assigned to the assignee of the present invention, means for homogenous liquid and gas distribution are of extreme importance.
The cost of high efficiency packing of the type set forth in the aforesaid patent commands attention to proper vapor-liquid distribution. Even small regions of non-homogenous interaction between the vapor and liquid is an expensive and wasteful loss not consistent with the utilization of high efficiency packing, where space and homogeneity in vapor-liquid interface are both expected and necessary for proper operation. High efficiency packing of the state of the art varieties set forth and shown in the aforesaid U.S. patent requires counter-current vapor-liquid flow through the channels defined by opposed corrugations of sheets disposed therein. If the initial liquid or gas distribution fails to enter a particular corrugation pattern, then precious surface area is lost in the packing until the liquid and vapor are urged to migrate into and interact through the unfilled regions of the packing. Only by utilizing proper vapor and liquid distribution means may effective and efficient utilization of high efficiency packing, as well as conventional dumped packing, be assured.
The development of systems for adequate liquid distribution in process towers has been limited as set forth above. In the main, it is known to discharge and distribute liquids with spray orifices, supply pipes, perforated plates, apertured troughs and nozzles. Gas is concomitantly discharged in an ascending turbulent configuration to provide adequate vapor distribution. Although many prior art systems are generally effective in distributing some vapor and some liquid to most portions of the packing bed, uniform distribution thereacross is usually not obtained without more sophisticated distribution apparatus. For example, unless gas is injected into a myriad of contiguous areas beneath the packing bed with equal pressure in each area, the mass flow of vapor upwardly through the packing bed will not be uniform. Random vapor discharge simply distributes unequal amounts of vapor across the lower regions of the packing bed but does not in any way assure equality in the distribution. Likewise the simple spray of liquid atop the packing bed, though intended to be effective in wetting all surface areas, often results in high concentrations of liquid flow in certain packing bed areas and less concentrations of liquid flow in others. This unevenness, of course, depends on the spray device. Orifice distributors are generally more susceptible to plugging than other types of distributors, and plugging tends to be non-uniform leading to uneven irrigation within the tower. Surface irregularities in a distributor pan occurring during manufacture will, likewise, increase flow resistance and induce liquid level gradients. With a level gradient, the fluid head between holes varies and the flow of fluid from those holes is not uniform. This is a distinct disadvantage. Any flow irregularity which focuses the flow in one area while reducing flow in other areas is deleterious.
Other structural and functional features of process columns can contribute to flow irregularity. These features include the size and shape of the distributor troughs, steps taken to maintain uniform fluid level in said troughs, and the means by which fluid is distributed to said troughs. The most conventional fluid distribution technique includes the use of a relatively large central channel called a parting box. The parting box is disposed above the distributor troughs and receives the initial flow of fluid from a supply pipe. The fluid enters the parting box and flows therefrom into the distributor troughs. If the parting box is located at one end of a very long distributor trough, as in a large diameter process tower, a fluid gradient can be created simply by virtue of the flow resistance imparted by the side walls of the trough and the discharge of liquid therefrom through the apertures in the trough. The result is reduced liquid flow rates from the ends of the trough and high liquid flow rates from the region of the trough near the parting box. Such liquid gradients resulting in loss of fluid head toward the ends of the parting box can be addressed in part by utilizing secondary parting boxes at intermediate positions between the center diameter of the tower and the ends of the elongate troughs. This does, however, require additional metal in fabrication as well as other structural and functional considerations in the manufacture thereof.
Structured packing can tolerate very little maldistribution, while dump packing can sustain relatively large variations in liquid distribution. Unfortunately, the manifestation of uneven liquid distribution often occurs in the vicinity of the most even, or uniform, vapor distribution. This is because vapor has had a chance to more evenly distribute through the packing bed prior to engaging the liquid distribution flow. It would be an advantage, therefore, to provide means for even liquid and vapor distribution prior to entry of said vapor as well as liquid into the packing bed and in a manner providing both a uniform spread of said liquid as well as vapor and uniform volumetric distribution thereof.
The present invention provides such an improved system of vapor-liquid distribution through a liquid distributor assembly incorporating primary distributor channels disposed within the parting box and/or certain distributor troughs. Primary liquid distribution may therein be provided in the parting box and/or troughs through decked or "piggy-back" channels which carry the primary liquid flow to thereby reduce both the potential fluid gradient and the variable liquid head that would ordinarily plague such systems. In this manner, the advantages of uniform flow distribution may be provided in an assembly facilitating a low profile configuration, minimum material costs and reduced labor costs such as welding, within the tower.